The invention is directed to a method of making fibrereinforced plastic molded parts.
When making fiber reinforced plastic molded parts it has proven advantageous to have a large proportion of fibers with a view to the obtainable strength of the parts. To this end a cold press molding method has been developed in which the fibrous material, after impregnation with resin, is subjected to high pressure in a press until the resin is cured, so that the resin will be squeezed out to a large extent. With this method the fiber content cannot be increased to more than about 35%. Above all, it is impossible with this method to make moulded parts including undercuts.
The U.S. Pat. No. 3,050,780 discloses a method by means of which fiber reinforced molded parts with undercuts may be made. Glass fiber material is placed in a hollow mould, thereupon a plastic composition is applied to the inside of the glass fiber material and is pressed into the same by means of a pressure bag so as to be distributed in the glass fiber material. When the plastic composition is cured, the pressure is removed from the pressure bag and the bag is withdrawn from the thus formed hollow body. In this way it is possible to produce hollow articles with undercuts. It is a drawback of this known method that the plastic composition will not be evenly distributed in the glass fiber material when the pressure bag is inflated. When being inflated, the pressure bag contacts the glass fiber material on all sides and compresses the same, so that the air entrapped in the glass fiber material can hardly escape, and for the synthetic resin the flow resistance in the glass fiber material also increases to such an extent that the resin is distributed only with difficulties. Consequently, the parts produced with this method cannot be made without defects but contain a large number of air bubbles or of locations to which no synthetic resin has penetrated. For this reason the known method cannot be used, for instance, to produce safety helmets, which are subject to stringent requirements in respect of safety and strength, as is the case for instance with motorcycle safety helmets. Therefore, the known method also has not been used for such safety helmets; the commercially available safety helmets have so far been produced as hand lay-up laminates with a high resin content, and they are therefore heavy and entail high material costs.
The known method according to the prior art (U.S. Pat. No. 3,050,780) has the further drawback that pressure build-up in the mold cannot be properly controlled, so that due to uneven pressure inside the mold the molded parts made therein also exhibit defects in which the glass fiber material has not been completely wetted by the synthetic resin composition.